1. Field of the Invention
The present invention relates to a communication system employing an Orthogonal Frequency Division Multiple Access (OFDMA) scheme, and more particularly to a subcarrier assignment apparatus and a subcarrier assignment method for site diversity in an OFDMA communication system.
2. Description of the Related Art
Considerable research has been pursued toward a 4th generation (4G) communication system, a next generation communication system, for providing users with services which have various Qualities of Service (QoS). In particular, research is being vigorously pursued to support high-speed services which ensure mobility and QoSs for a broadband wireless communication system such as a Wireless Local Area Network (WLAN) communication system and a Wireless Metropolitan Area Network (WMAN) communication system.
In the 4G communication system, public attention is attracted to an OFDM scheme which is useful for high-speed data transmission over a wired/wireless channel. The OFDM scheme is a data transmission scheme using a multi-carrier and is also a kind of a Multi-Carrier Modulation (MCM) scheme in which symbol strings inputted in series are converted into parallel symbol strings, and the respective parallel symbol strings are modulated with a plurality of subcarriers having mutual orthogonality and then transmitted.
Wideband spectrum resources are required for providing high-speed and high-quality wireless multimedia services by the 4G communication system. However, when the wideband spectrum resources are used, the fading phenomenon on a wireless transmission path becomes serious due to multipath propagation, and a frequency-selective fading phenomenon, which may also occur even in a transmission band. Thus, in implementing the high-speed wireless multimedia services, an OFDM scheme having very strong resistance to the frequency-selective fading phenomenon gains an advantage over a conventional Code Division Multiple Access (CDMA) scheme. Thus, there is a strong tendency to utilize the OFDM scheme in the 4G communication system.
In general, a wireless communication system is a cellular communication system consisting of a plurality of cells, in which a Base Station (BS) may control the plurality of cells and a Mobile Station (MS) is connected to the BS to communicate therewith. Also, a BS in communication with the MS, (i.e., a BS providing services to the MS) is a serving BS, and the serving BS manages various resources for supporting the MS's communications. As a result, the serving BS is a BS which supports all operations for performing communications with the MS until the MS handovers to any BS other than the serving BS, such as any neighbor BS (i.e., target BS). The MS has only one serving BS.
In such a cellular configuration, different characteristics are exhibited according to whether the characteristics corresponds to those of a cell center region or those of a cell boundary region as will be described below:
(1) in regard to delay spread, the cell center region has comparatively small delay spread, but the cell boundary region has comparatively large delay spread.
(2) with respect to frequency selective fading, the cell center region is stable without substantial influence of the frequency selective fading, but the cell boundary region is seriously influenced by the frequency selective fading; and
(3) in view of Inter Cell Interference (ICI) variation, the cell center region has almost no ICI variation and thus is stable, but the cell boundary region shows significant ICI variation.
The 4G communication system aims at providing high-speed and high-quality services, so it must be able to provide the high-speed and high-quality services to the MS operating in the cell center region, as well as the MS operating in the cell boundary region in the same way. However, since the channel qualities of the cell boundary region are generally inferior to those of the cell center region as stated above, there is an important objective to compensate for the deterioration in channel quality of the cell boundary region.